Most radar-beacon interrogators operate on preassigned interrogation-repetition frequencies. These frequencies are chosen so that interference among interrogators that are geographically capable of interrogating the same transponder simultaneously will not be excessive. When more than a few interrogators are operating in the same geographic area, interrogation-repetition-frequency (IRF) assignments must be made carefully, and the interrogators must maintain precise control over their interrogation rates.
It is no longer essential that a radar-beacon, or secondary radar, system be interconnected with a primary radar system. In fact, many of today's radar-beacon systems operate independently of primary radar, and in some cases the radar-beacon system has completely replaced the primary radar. Thus, whether the primary radar system operates at a fixed pulse-repetition-frequency or uses prf jittering, the radar-beacon system associated with it can operate at any desired IRF, and this frequency can be crystal controlled if desired.
A radar beacon system operates on the master-slave principle. The interrogator, as master, demands replies. The transponder, as slave, replies dutifully unless prevented from doing so by circumstances beyond its control. As in any master-slave situation, difficulties arise when two or more masters make conflicting demands upon the same slave. At present most interrogators demand far more replies from most transponders than they really need, or should really need if proper use were made of replies received. Why radar-beacon users insist on continuing to use high interrogation-repetition rates is easy to understand. To provide safe air traffic control the controllers must receive a sufficient number of replies from all transponders within range, not just from those which happen to receive excessive interrogations. If interrogation repetition frequencies were reduced so that nearby transponders would receive only as many interrogations as needed, then some of the more distant transponders would most certainly not receive enough. Since each interrogation may be received by both nearby and distant transponders, there is no way for the interrogator to reduce interrogation rates enough to prevent many, in fact most, transponders from receiving far more interrogations than necessary.
However, if it can be determined how many replies each interrogator actually needs from a transponder during each scan of its antenna past that transponder, and the transponder can recognize the source of each sequence of interrogations, then the transponder can be made to limit its replies so that no interrogator can obtain more than the number of replies it needs. The number of replies actually needed per scan past a transponder is called an adequate response, and reply limiting that permits this number but only this number to each interrogator, is reply limiting based on the adequate response principle.
A transponder might recognize interrogators in several ways. Military users of radar beacon systems have been concerned for some time about how to recognize friendly interrogators versus enemy ones, so that transponders could be made to reply only to the former. During World War II, and in every conflict since, transponders have been turned off in certain areas, so that enemy interrogators could not get replies from them. The separation of interrogators into those that are friendly and those that are not, although useful for other purposes, does nothing to reduce the number of replies each of our transponders must transmit every time a friendly interrogator's antenna sweeps past it.
If every interrogator had its own personal identification code, such as every military transponder now has, then transponders might use this code to determine when each interrogator had elicited and adequate number of replies. The interrogator personal identification (IPI) code would, of course, have to be transmitted with every interrogation. There are a number of ways in which IPI codes might be used for reply limiting, and they offer some unique capabilities in other areas.
If the interrogation repetition periods of interrogations received are recognized by a transponder and this information is then used to limit replies, the adequate-response type of reply limiting is possible without increasing the interrogation length or frequency spectrum used by interrogations.